In recent years, there have been remarkable developments in the field of information processing, such as personal computers, as well as in the field wireless communication, such as mobile phones. In these fields, improvement in information processing speed requires for circuit boards to increase propagation speed and reduce transmission loss at high frequency ranges. In general, circuit boards are produced by impregnating a glass cloth with an epoxy resin having a low dielectric constant to obtain an insulating layer; piercing through the insulating layer from top side to bottom side by drilling or laser irradiation to form a through-hole, followed by copper-plating to make an electrical pathway; patterning the surface thereof to form a wiring substrate; and further laminating a plurality of wiring substrates together as a multilayer circuit, and/or covering the wiring substrate or the multilayer circuits with a coverlay having an insulating property.
Conventionally, development in communication equipment using radio waves, such as mobile phones, wireless LANs, devices for determining inter-vehicle distance, has promoted use of communication signals in higher frequency ranges, as well as miniaturization of devices, in particular along with information of large capacity and high-speed.
The circuit boards for such communication equipment used in a high frequency range require insulating materials having a low dielectric constant and a low dielectric loss tangent. In addition, from the viewpoint of solder heat resistance, substrates comprising a fluorocarbon resin, a BT (bismaleimide-triazine) resin or the like have been used as a base material.
Further, thermoplastic liquid crystal polymer films have come to be used from the viewpoint of heat resistance, dimensional stability, good interlayer adhesion in a multilayer structure, low water absorption, and low transmission loss. The thermoplastic liquid crystal polymer films, however, cannot to be molded by using equipment for multilayer production conventionally used for thermosetting resins because the thermoplastic liquid crystal polymer films require to be heat-treated at a temperature of 250° C. or higher in multilayer lamination process, resulting in requirement for new equipment investment.
Furthermore, films used as coverlay require insulation property, heat resistance, and dimensional stability. For example, a polyimide film is used as a coverlay via adhesive to constitute a flexible circuit board; while rigid circuits use solder resist ink. In recent years, in order to reduce transmission loss in high frequency band, a method of using a thermoplastic liquid crystal polymer film as a coverlay has come to be proposed.
Under these circumstances, there have been disclosed multilayer laminates comprising a film made of a thermoplastic liquid crystal polymer being capable of forming an optically anisotropic melt phase, and a circuit layer(s) including an electrical wiring(s), both of which are bonded by thermal compression (for example, see Patent Documents 1 and 2). However, production of multilayer laminates disclosed in Patent Documents 1 and 2 requires molding at a high temperature as high as around the melting point of the thermoplastic liquid crystal polymer, which is much higher than 200° C. Accordingly, there is a problem that the specific press machine specialized in high temperature application must be used.
In order to solve the above problem, a multilayer wiring board has been proposed. The multilayer wiring board is produced by plasma treating the surface of a liquid crystal polymer film layer to enhance surface adhesion, thereafter forming a coating layer composed of a polyphenylene ether-based resin on the top and bottom surfaces of the liquid crystal polymer film layer (see, for example, Patent Document 3). However, the multilayer wiring board in Patent Document 3 is produced by covering the plasma-treated surfaces of the liquid crystal polymer film layer on top and bottom sides with polyphenylene ether-based covering layers followed by forming wiring circuits on each surface of the polyphenylene ether-based covering layers. As a result, the circuit board may have lowered dimensional stability and poor heat resistance after lamination. As another problem, the multilayer wiring board cannot be sufficiently resistant to high heat soldering.
There has been proposed a high-frequency circuit substrate having a low dielectric constant and a low dielectric loss tangent in high frequency ranges, as well as having excellent solder heat resistance and improved bonding to a conductor in intimate contact (see, for example, Patent Document 4). However, since the circuit substrate is formed of a thermoplastic resin composition containing a polyphenylene ether-based resin and a thermoplastic resin including a liquid crystal polymer so as to produce a film by melt-kneading the thermoplastic resin composition with an inorganic filler and additional constituents, followed by extruding the melt-kneaded resultant to form a film, the film has ununiform dielectric loss tangent and dielectric constant depending on the alloy state and dispersion state of the individual resins. As a result, the circuit substrate has problems of not only ununiform transmission loss, but also poor solder heat resistance and dimensional stability after lamination.
Furthermore, a coverlay film having a low dielectric constant and a low dielectric loss tangent has been proposed for the purpose of reducing transmission loss in high frequency (for example, see Patent Document 5). Patent Document 5 proposes a film formed from a vinyl compound, a polystyrene-poly(ethylene/butylene) block copolymer, an epoxy resin, and a curing catalyst. The film is excellent in electrical characteristics in high frequency, but has a problem that the film does not have satisfactory solder heat resistance as a circuit substrate.
Moreover, a high-frequency circuit board has been proposed (see, for example, Patent Document 6). The high-frequency circuit board has a low dielectric constant and a low dielectric loss tangent at high frequency region, as well as is excellent in solder heat resistance and bonding to a conductor in intimate contact. However, in a thermoplastic resin composition containing a polyphenylene ether-based resin and a thermoplastic resin including a liquid crystal polymer as a circuit material proposed by Patent Document 6, there has been a problem that the film is poor in solder heat resistance and dimensional stability after lamination.
Furthermore, the rapid development of electronic devices in recent years requires mounting of printed circuit boards with very high performance. As a way to meet these requirements, demand for rigid-flexible circuit boards has expanded. The rigid-flexible circuit board comprises a rigid board and a flexible board; both boards are laminated and integrated with each other. Such a rigid-flexible circuit board makes it possible to mount a component on a rigid part having rigidity comparable to rigid board as well as to achieve bendable solid interconnection at a flexible part having bendability. Accordingly, the rigid-flexible circuit board realizes miniaturization, weight reduction, portable application, and densification with respect to electronic device.
For example, Patent Document 7 describes a structure comprising a flexible part and a rigid part. The flexible part comprises a base film made of a heat-resistant resin such as a polyimide with metal conductors such as copper foils disposed on both surfaces of the base film, an acrylic adhesive, and a coverlay, wherein the base film with the metal conductors and the coverlay are successively laminated via the acrylic adhesive; the rigid part comprises one or more portions of the flexible part, a copper-clad laminate and a prepreg disposed therebetween, wherein the prepreg is obtained by impregnating a glass base material with an epoxy or others.
In order to meet requirements in high signal speed as well as high density, Patent Document 8 discloses a rigid-flexible circuit board comprising a flexible part made of a polyimide resin film to form a circuit layer, and a rigid part comprising a glass fiber base material and a polyimide resin film to be thermo-compression bonded to a portion of the flexible part. The polyimide resin film constituting the flexible part is covered with a covering layer via a thermosetting adhesive. This circuit board is excellent in bendability of the flexible part, and capable of meeting demand for high density by reducing thickness of the circuit board.
However, the polyimide films used in Patent Documents 7 and 8 have disadvantage that the polyimide is large in dielectric loss tangent and has high transmission loss although the polyimide films are used from the viewpoint of high heat resistance. As a result, noise easily occurs in the circuit boards when transmitting high-frequency signals.